Field of the Invention
The present invention relates to a sheet-metal foil for a metallic honeycomb body which includes sheet-metal layers and a plurality of the sheet-metal foils being joined to form the honeycomb body by using a brazing material in powder form. The invention also relates to a honeycomb body, in particular as a catalyst carrier body in an exhaust system of an internal combustion engine, which includes at least partially structured sheet-metal layers that are wound and/or stacked in such a way as to form passages through which a fluid can flow. The sheet-metal layers are at least partially joined to one another through the use of a brazing material in powder form. The invention further relates to a process for producing a honeycomb body.
The use of metallic honeycomb bodies which have a plurality of sheet-metal foils that are wound and/or stacked to form the honeycomb structure is known, in particular, in the form of a catalyst carrier body in exhaust systems of spark-ignition or diesel engines used in the automotive industry. The sheet-metal foils are joined to one another using a brazing material in order to ensure structural integrity of the honeycomb body even under the thermal and dynamic loads which are encountered in an exhaust system. In addition to brazing material in wire form or ribbon form, it is also possible for brazing material in powder form to be used as the brazing material.
In order to ensure long-term functionality of the honeycomb body, fixing of the metallic honeycomb body in the exhaust system must be such that the components of the honeycomb body are permitted to undergo different degrees of thermal expansion, if necessary. That is especially important in particular with a view toward the sheet-metal foils being joined to a tubular casing which surrounds the honeycomb body. For that purpose it is known, for example, for the honeycomb body to be joined to the tubular casing only in an axial sub-region which is preferably disposed over an area of 20 mm, starting from the exhaust-gas inlet side. In that way, a relative movement between the sheet-metal foils, which heat up quickly and cool down quickly, and the tubular casing, is ensured in the sub-regions in which the honeycomb body is not joined to the tubular casing, and the occurrence of thermal stresses is prevented. In that context, it is particularly important for it to be possible to accurately comply with the desired limits with regard to the joining region during the production process. That does not present major problems when using brazing material in wire or ribbon form, which ensures that the brazing material is held together to a certain degree.
However, if the production process using brazing material in powder form is considered, it has not heretofore been possible to ensure a tightly delimited and defined area of use with a sufficient degree of process reliability, in particular for series or mass production. The fundamental reason therefor is in particular that before the sheet-metal foils are wound and/or stacked, an adhesive which is used to fix the brazing material in powder form is applied in the joining region. In particular during the operation of winding the sheet-metal foils, the adhesive becomes locally smeared and/or winding or stacking errors occur due to the significantly increased friction caused by the adhesive. The result of such smearing and/or errors is that brazing material sticks in areas of the honeycomb body where such sticking is undesirable, in particular including outside the joining region which has previously been defined.